Decomposition

Although there have been many decomposition studies conducted in different parts of the world and under different environmental conditions, most studies have been conducted in temperate areas and fewer in tropical and subtropical habitats. Common to the majority of these studies has been an attempt to divide the decompositional process into a series of discrete stages. Decomposition is, in nature, a continuous process and so discrete combinations of physical parameters and arthropod assemblages do not occur. There is a value to these stages, however, in providing reference points when faced with the problem of explaining the events associated with decomposition to a jury.

Regardless of locality, there are certain common patterns. The faunas involved tend to be regional, except for some widely distributed species of Diptera and Coleoptera, but the families involved are somewhat stable. The division of decomposition into five stages can be applied to most studies.

Fresh Stage

This stage begins at the moment of death and ends when bloating is first evident. The first insects to arrive at the corpse are flies in the families Calliphoridae and Sarcophagidae. Adult females investigate the corpse, frequently feed, and then, depending on the species of fly, deposit either eggs or larvae around the natural body openings associated with the head (eyes, nose, mouth, and ears) and anogenital regions. Wounds are secondary sites of attraction to tropical species but also may be of major significance in temperate environs.

Bloated Stage

Putrefaction, the principal component of decomposition, begins. Gases produced by the metabolic activities of anaerobic bacteria first cause a slight inflation of the abdomen and, later, the corpse appears balloon-like and fully inflated. Internal temperatures rise during this stage as the result of bacterial decay and metabolic activities of feeding dipteran larvae. Calliphoridae are strongly attracted to the corpse during this stage. As the corpse inflates, fluids are forced from natural body openings and seep into the soil. These fluids combined with the by-products (ammonia, etc.) produced by the metabolic activities of the dipteran larvae cause the soil beneath the corpse to become alkaline and the normal soil fauna departs.

FIGURE 1 Pig carcass during decay stage of decomposition. (A) Day 8: active maggot mass consists primarily of third instar Chrysomya rufifacies. (B) Day 13 (end of decay stage): maggots have completed development and migrated away from carcass for pupariation.

Decay Stage

This is the only stage in the decomposition process which has a distinct starting point. The decay stage begins when the skin is broken, allowing gases to escape and the corpse to deflate. Dipteran larvae form large feeding masses and are the predominant taxa present (Fig. 1A). Although some predatory forms, such as beetles, wasps, and ants, are present during the bloated stage, both necrophagous and predatory taxa are observed in large numbers during the later portions of the decay stage. By the end of this stage, most Calliphoridae and Sarcophagidae have completed their development and departed the corpse for pupariation (Fig. 1B). Dipteran larvae will have removed most of the soft tissue from the corpse by the end of the decay stage.

Postdecay Stage

As the remains are reduced to skin, cartilage, and bone, Diptera cease to be predominant. In xerophytic and mesophytic habitats, various Coleoptera predominate throughout this stage, and the diversity of these taxa increases. Associated with this increase is an increase in the numbers of parasites and predators of beetles. In wet habitats (swamps, rain forests, etc.), however, other taxa, primarily Diptera, and their predator/parasite complexes predominate.

Skeletal Stage

This stage is reached when only bones and hair remain. No obvious carrion-frequenting taxa generally are present and there is a gradual return of the normal soil fauna to the area under the corpse. An examination of the soil during the early portions of this stage will reveal various acarine groups that may be of use in estimating the postmortem interval. There is no definitive end to this stage and changes in the soil fauna may be detectable months or even years following the death.